The present invention relates in general to the measurement of velocity of a fluid inside a conduit. More particularly, the invention relates to an electro-acoustic flowmeter for measuring fluid velocity in a more accurate manner than known hereinbefore.
The principle of deriving the velocity of a fluid (liquid, gas, slurry, blood) inside of a conduit (pipe, tube, vessel, trough) from the measurement of the differential time delay of upstream and downstream sound or ultrasound is known. See for example, Noble, F. W., The Review of Scientific Instruments, Volume 39, No. 9, September, 1968, pages 1327-1331.
The upstream and downstream transducers, also referred to herein and shown in FIGS. 1 and 2, are typically located on a line intersecting the flow axis obliquely. Described hereinafter are different arrangements for the placement of these transducers. The embodiments provided are either invasive or non-invasive.
Flowmeter design is beset by the problem of soundpaths outside the fluid, sometimes in the nature of reverberations. The following inventors have devised certain systems to overcome these problems.
For example, pulsed operation has been proposed by Cirulis, U.S. Pat. No. 3,720,100, Gassmann, U.S. Pat. No. 4,069,713, and others. While pulsed operation resolves different paths of differential delays substantially greater than pulse width, the latter cannot be narrower than the reciprocal bandwidth (in radians/sec.) of the system. Since acoustic transducers are narrowband structures, the desired resolution is limited at radio frequencies to substantial values of differential path delay. Thus, such systems may suffer from practical limitations on transducer separation.
Another technique is material matching such as proposed by Krylova, U.S. Pat. No. 4,065,958. Yet another technique is shown by Lynnworth, U.S. Pat. No. 4,004,461 which depicts an arrangement of acoustically mismatched pipe sections. Both of these techniques are used in conjunction with collimated beams. Again, the acoustic mismatching of material may not be practical where it creates other problems depending upon the particular environment. Furthermore, these techniques are proposed on the theory of essentially ignoring the signals within the walls of the conduit.
The interferometric principle of the present invention is tolerant of interference by stationary sound paths and by electromagnetic coupling between exciter and receiver; it is compatible with rather broad pulsing which serves to reduce the effect of long period reverberation. An essential part of the present invention is a repetitive sweep of the excitation frequency. In this regard, reference is made to the Franklin U.S. Pat. No. 3,568,661 which discloses simultaneous swept frequency excitation and reception at each site. Each of the transducers is connected to its driver and mixer-bandpass amplifier arrangement, both of the latter feeding a phase detector with the output thereof stated as being a measure of velocity. Although Franklin neither mentions nor claims frequency measurement, the "measure" is a frequency or period count by inference. Since the transducers of his invention do not invade the conduit (such as a blood vessel), his disregard of signals in the wall is unrealistic. As is shown in the mathematical theory below, only in the case of wall signals which are negligibly small compared with signals in the fluid is the frequency of a component in the phase detector output proportional to velocity magnitude, said frequency being given by: EQU f(.tau..sub.u -.tau..sub.d)
where
f=rate of change of frequency, and PA1 .tau..sub.d, .tau..sub.u =downstream (upstream) delay.
Since the detector output component of frequency f (.tau..sub.d +.tau..sub.u) has the same amplitude, one would then have to provide means for suppressing this component. However, this is not at all mentioned in Franklin and may not be practical in his apparatus. The numerical example given by Franklin is for a beat frequency of 10 Hz (maximum) and a resolution of 10 nanoseconds for the differential path delay, necessitating f=10.sup.9 Hz/sec. for at least 0.1 sec., which corresponds to a deviation of 100 MHz. Franklin makes no statement concerning the response of transducers which resonate about 10 MHz to a 110 MHz signal. The Franklin invention has certain drawbacks associated therewith and may in fact not provide accurate and practical results.